The principle of radar is well known. There exist a variety of configurations of transmitters, receivers, antennas, transmitted waveforms and signal processing techniques, which have been adapted to provide for systems for detection of an aircraft, vehicle, or object, weather or other natural phenomenon, and the like. Each of the configurations may take into account such factors as the range to the target, target size and speed of motion, response time, and the desired resolution in range, speed and direction.
FIG. 1 illustrates a frequency-modulated continuous-wave (FMCW) radar 1 having a transmitting antenna 2, a receiving antenna 3, and a power divider 4, or other circuit to provide a sample of the transmitted signal to a mixer 5. A waveform generator 6 provides the transmitted signal waveform. The transmitted signal is reflected from a target 9. The reflected signal is received by the receiving antenna 3 and mixed with the sample of the transmitted signal in the mixer 5. A difference frequency between the generated waveform and the reflected signal is formed by the mixer 5 and amplified and filtered in the remainder of the receiver 7. A receiver may include a mixer 5, and filters and amplifiers 8 to select and amplify an appropriate mixer output. A signal processor 10 extracts a desired radar response associated with a target.
FIG. 2 illustrates the relative relationship of a transmitted signal 11, and a received signal 12, as a function of time during a linear FMCW transmitted signal period. The linear FMCW transmission is repeated periodically, and, except for the period of time associated with the transition from the upper frequency to the lower starting frequency of the transmitted waveform, a difference frequency between the transmitted waveform and the received waveform is given by:Δf=KΔt+fd  (1)
The time rate of change of frequency or ramp rate, K, may be a positive or negative quantity. The duration of a ramp, TRamp, and the ramp rate K, determine the time-bandwidth product of the FMCW signal. The range resolution of the radar is approximately 1/(TRampK), which is the inverse of the transmitted bandwidth of the entire ramp, and fd is the target Doppler shift frequency. Sequences of ramps may have the same ramp rate, K, and duration, TRamp, or sequences may have values of K and TRamp which are different from ramp to ramp, depending on the type of signal processing employed and the particular information that is to be extracted from the received signal.
The receiver 7 may be of the homodyne or superheterodyne type, as is known in the art, with a signal output spectrum having a frequency content given by (1), with the receiver signal output as an input to the signal processor 10. Frequency components associated with the transition between the upper and lower frequency limits of the ramp at the end of a ramp period are usually substantially greater that the maximum difference frequency expected for the radar design, and are eliminated by filtering in the receiver 7 or signal processor 10.
The signal output of the receiver 7 is processed by a signal processor 10 to derive the required output data. In a situation where the target and the radar are stationary with respect to each other, the output of the receiver 7 is a frequency whose value is a function of the distance to the target and the radar parameters in accordance with the first term on the right hand side of (1). The signal processor may include a spectrum analyzer, frequency counter, a computer executing a Fast Fourier Transform (FFT) or the like. When there is relative motion between the radar and the target, a Doppler frequency shift in accordance with the second term on the right hand side of (1), and depending on the magnitude of the Doppler shift and its relevance to the use of the radar, a number of means of separating the Doppler shift from the range-dependent frequency component are known in the art.
One use of radar is in automotive applications relating to safety or operator alerting, such as determining the distance to obstacles or other vehicles, or the closing speed, and the direction of the closing object. It is desirable to perform these functions with a minimum of expense, and in a manner compatible with the installation constraints associated with vehicular applications. The FMCW waveform has been used in this application, in configurations which use two antennas: a transmitting antenna and a receiving antenna. Use of two antennas increases the isolation between the transmitted waveform and the received waveform to avoid such problems as receiver overload, transmitted noise desensitization, or the like. In another configuration, a single antenna has been used for transmitting and receiving, with the isolation between the transmitting signal and the received receiver input being obtained by the use of a circulator. However, in some circumstances, the isolation of the transmitted signal from the receiver input may be inadequate, the dynamic range of the receiver may not be sufficient to prevent overload by signal returns from nearby objects, or the transmitted background noise coupled through the circulator may exceed the desired signal amplitude.
An improved means of providing a radar for automotive and other uses is desired.